The invention relates to catalysts useful for olefin polymerization. In particular, the invention relates to xe2x80x9csingle-sitexe2x80x9d catalysts that incorporate at least one anionic heterocyclobutenyl ligand.
Interest in single-site (metallocene and non-metallocene) catalysts continues to grow rapidly in the polyolefin industry. These catalysts are more reactive than Ziegler-Natta catalysts, and they produce polymers with improved physical properties. The improved properties include narrow molecular weight distribution, reduced low molecular weight extractables, enhanced incorporation of a-olefin comonomers, lower polymer density, controlled content and distribution of long-chain branching, and modified melt rheology and relaxation characteristics.
Traditional metallocenes commonly include one or more cyclopentadienyl groups, but many other ligands have been used. Putting substituents on the cyclopentadienyl ring, for example, changes the geometry and electronic character of the active site. Thus, a catalyst structure can be fine-tuned to give polymers with desirable properties. Other known single-site catalysts replace cyclopentadienyl groups with one or more heteroatomic ring ligands such as boraaryl (see, e.g., U.S. Pat. No. 5,554,775), pyrrolyl, indolyl, (U.S. Pat. No. 5,539,124), or azaborolinyl groups (U.S. Pat. No. 5,902,866).
Single-site catalysts typically feature at least one polymerization-stable, anionic ligand that is purely aromatic, as in a cyclopentadienyl system. All five carbons in the planar cyclopentadienyl ring participate in bonding to the metal in xcex7-5 fashion. The cyclopentadienyl anion functions as a 6xcfx80-electron donor. Similar bonding apparently occurs with heteroatomic ligands such as boratabenzenyl or azaborolinyl.
In contrast, olefin polymerization catalysts that contain heterocyclobutenyl ligands are not known. The neutral ligand precursors can be prepared by known literature procedures.
In spite of the availability of synthetic routes to heterocyclobutenyl anions, their use as ligands for metallocene or single-site catalysts for olefin polymerization has not been suggested. Organometallic complexes from these ligands would provide a new class of potentially valuable catalysts to polyolefin producers.
The invention is a single-site olefin polymerization catalyst. The catalyst comprises an activator and an organometallic complex. The organometallic complex comprises a Group 3 to 10 transition metal, M, and at least one anionic heterocyclobutenyl ligand that is bonded to M.
Evidence from molecular modeling studies indicates that single-site catalysts based on anionic heterocyclobutenyl ligands will exhibit improved stability versus catalysts based on cyclopentadienyl and substituted cyclopentadienyl ligands. This improved stability should impart increased catalyst efficiency, especially at higher process temperatures.
Also provided is a two-step method of producing the catalyst. Step one involves deprotonating a heterocyclobutene and reacting the resulting anion with a Group 3 to 10 transition metal source to produce an organometallic complex comprising the metal, M, and at least one heterocyclobutenyl ligand that is bonded to M. In step two, the product is combined with an activator. The ease and inherent flexibility of the synthesis puts polyolefin makers in charge of a new family of single-site catalysts.
Catalysts of the invention comprise an activator and an organometallic complex. The catalysts are probably xe2x80x9csingle sitexe2x80x9d in nature, i.e., they are distinct chemical species rather than mixtures of different species. They should give polyolefins with characteristically narrow molecular weight distributions (Mw/Mn less than 3) and good, uniform comonomer incorporation.
The organometallic complex includes a Group 3 to 10 transition metal, M. As used herein, xe2x80x9ctransition metalxe2x80x9d includes metals of the lanthanide and actinide series. More preferred complexes include a Group 4 to 6 transition metal; most preferably, the complex contains a Group 4 metal, i.e., titanium, zirconium or hafnium.
The organometallic complex also comprises at least one heterocyclobutenyl anion that is bonded, most likely xcfx80-bonded, to the metal. By xe2x80x9cheterocyclobutenyl anion,xe2x80x9d we mean an anion formed from a four-membered cyclobutene ring where one of the saturated carbons in the cyclobutene is replaced with a heteroatom.
The heterocyclobutenyl anions are usually generated from the corresponding neutral compounds by deprotonation with a potent base as is described in more detail below. The synthesis of phosphacyclobutenes (dihydrophosphetes) from the corresponding titanacyclobutene is known (see K. Doxsee et al., J. Am. Chem. Soc. 111 (1989) 9129), and azacyclobutenes (azetines) should be available from the same method. Alternatively, azetines should be available from a method analogous to that described in the literature for 1-acyl-2-azetines (see M. Jung et al., J. Org. Chem. 56 (1991) 6729), where an azetidonol is mesylated, and then the mesylate is treated with base to eliminate methanesulfonic acid and afford the azetine. The synthesis of thiacyclobutenes (thietes) by facile Hofmann elimination of the 3-aminothietane derivatives has been described (see D. Ditmer et al., J. Org. Chem. 37 (1972) 1111). The chemistry of oxetenes has been reviewed (see R. Linderman, Compr. Heterocycl. Chem. II (1996), 1B 721-753, Editor A. Padwa, Elsevier Publishers Oxford, UK).
The heterocyclobutenyl anion may be bridged to another ligand, which may or may not be another heterocyclobutenyl anion. Preferred heterocyclobutenyl anions have the general structure: 
where A is N, P, O or S. R is C1-C30 hydrocarbyl or trialkylsilyl. When A is N or P, n is 1; when A is O or S, n is 0; each R1 is independently selected from the group consisting of R, H, Cl, and Br. When A is N, the anion is said to be an azetinyl anion. When A is P, the anion is said to be a phosphetyl anion. When A is S, the anion is said to be a thietyl anion and when A is O, the anion is an oxetenyl anion.
Exemplary anions are: 
The organometallic complex optionally includes one or more additional polymerization-stable, anionic ligands. Examples include substituted and unsubstituted cyclopentadienyl, fluorenyl, and indenyl, or the like, such as those described in U.S. Pat. Nos. 4,791,180 and 4,752,597, the teachings of which are incorporated herein by reference. A preferred group of polymerization-stable ligands are heteroatomic ligands such as boraaryl, pyrrolyl, indolyl, indenoindolyl, quinolinoxy, pyridinoxy, and azaborolinyl as described in U.S. Pat. Nos. 5,554,775, 5,539,124, 5,637,660, 5,902,866 and 6,232,260, the teachings of which are incorporated herein by reference. The organometallic complex also usually includes one or more labile ligands such as halides, alkyls, alkaryls, aryls, dialkylaminos, or the like. Particularly preferred are halides, alkyls, and alkaryls (e.g., chloride, methyl, benzyl).
The heterocyclobutenyl anions and/or polymerization-stable ligands can be bridged. For instance, a xe2x80x94CH2xe2x80x94, xe2x80x94CH2CH2xe2x80x94, or (CH3)2Si bridge can be used to link two heterocyclobutenyl anions or a heterocyclobutenyl anion and a polymerization-stable ligand. Groups that can be used to bridge the ligands include, for example, methylene, ethylene, 1,2-phenylene, and dialkyl silyls. Normally, only a single bridge is included. Bridging changes the geometry around the transition metal and can improve catalyst activity and other properties such as comonomer incorporation.
The organometallic complex preferably has the general structure: 
where M is a transition metal and A is N, P, O or S. R is C1-C30 hydrocarbyl or trialkylsilyl. When A is N or P, n is 1; when A is O or S, n is 0; each R1 is independently selected from the group consisting of R, H, Cl, and Br. Each L is independently halide, alkoxy, siloxy, alkylamino or C1-C30 hydrocarbyl. Lxe2x80x2 is substituted or unsubstituted cyclopentadienyl, indenyl, fluorenyl, boraaryl, pyrrolyl, indolyl, indenoindolyl, quinolinoxy, pyridinoxy or azaborolinyl; y is 0 or 1; and x+y satisfies the valence of M.
The catalysts include an activator. Suitable activators ionize the organometallic complex to produce an active olefin polymerization catalyst. Suitable activators are well known in the art. Examples include alumoxanes (methyl alumoxane (MAO), PMAO, ethyl alumoxane, diisobutyl alumoxane), alkylaluminum compounds (triethylaluminum, diethyl aluminum chloride, trimethylaluminum, triisobutyl aluminum), and the like. Suitable activators include acid salts that contain non-nucleophilic anions. These compounds generally consist of bulky ligands attached to boron or aluminum. Examples include lithium tetrakis(pentafluorophenyl)borate, lithium tetrakis(pentafluorophenyl)aluminate, anilinium tetrakis(pentafluorophenyl)borate, and the like. Suitable activators also include organoboranes, which include boron and one or more alkyl, aryl, or aralkyl groups. Suitable activators include substituted and unsubstituted trialkyl and triarylboranes such as tris(pentafluorophenyl)borane, triphenylborane, tri-n-octylborane, and the like. These and other suitable boron-containing activators are described in U.S. Pat. Nos. 5,153,157, 5,198,401, and 5,241,025, the teachings of which are incorporated herein by reference.
Suitable activators also include aluminoboronatesxe2x80x94reaction products of alkyl aluminum compounds and organoboronic acidsxe2x80x94as described in U.S. Pat. Nos. 5,414,180 and 5,648,440, the teachings of which are incorporated herein by reference.
The amount of activator needed relative to the amount of organometallic complex depends on many factors, including the nature of the complex and activator, the desired reaction rate, the kind of polyolefin product, the reaction conditions, and other factors. Generally, however, when the activator is an alumoxane or an alkyl aluminum compound, the amount used will be within the range of about 0.01 to about 5000 moles, preferably from about 0.1 to about 500 moles, of aluminum per mole of M. When the activator is an organoborane or an ionic borate or aluminate, the amount used will be within the range of about 0.01 to about 5000 moles, preferably from about 0.1 to about 500 moles, of activator per mole of M.
The activator is normally added to the reaction mixture at the start of the polymerization. However, when a supported catalyst system is used, the activator can be deposited onto the support along with the organometallic complex.
The catalyst systems are optionally used with an inorganic solid or organic polymer support. Suitable supports include silica, alumina, silica-aluminas, magnesia, titania, clays, zeolites, or the like. The support is preferably treated thermally, chemically, or both prior to use to reduce the concentration of surface hydroxyl groups. Thermal treatment consists of heating (or xe2x80x9ccalciningxe2x80x9d) the support in a dry atmosphere at elevated temperature, preferably greater than about 100xc2x0 C., and more preferably from about 150xc2x0 C. to about 600xc2x0 C., prior to use. A variety of different chemical treatments can be used, including reaction with organo-aluminum, -magnesium, -silicon, or -boron compounds. See, for example, the techniques described in U.S. Pat. No. 6,211,311, the teachings of which are incorporated herein by reference.
The complex and activator can be deposited on the support in any desired manner. For instance, the components can be dissolved in a solvent, combined with a support, and stripped. Alternatively, an incipient-wetness technique can be used. Moreover, the support can simply be introduced into the reactor separately from the complex and activator.
The loading of complex on the support varies depending upon a number of factors, including the identities of the complex and the support, the type of olefin polymerization process used, the reaction conditions, and other concerns. Usually, the amount of complex used is within the range of about 0.01 to about 10 wt. % of transition metal based on the amount of supported catalyst. A more preferred range is from about 0.1 to about 4 wt. %.
The invention includes a method for making the organometallic complex. The method comprises deprotonating a heterocyclobutene with at least one equivalent of a potent base such as lithium diisopropylamide, n-butyllithium, sodium hydride, a Grignard reagent, a combination of n-butyllithium and potassium t-butoxide or the like. The resulting anion is reacted with a Group 3 to 10 transition metal source to produce an organometallic complex. The complex comprises the metal, M, and at least one heterocyclobutenyl anionic ligand that is bonded, and probably xcfx80-bonded, to the metal. Any convenient source of the Group 3 to 10 transition metal can be used. Usually, the source is a complex that contains one or more labile ligands that are easily displaced by the heterocyclobutenyl anion. Examples are halides (e.g., TiCl4, ZrCl4), alkoxides, amides, and the like. The metal source can incorporate one or more of the polymerization-stable anionic ligands described earlier. The organometallic complex can be used xe2x80x9cas is.xe2x80x9d Often, however, the complex is converted to an alkyl derivative by treating it with an alkylating agent such as methyllithium. The alkylated complexes are more suitable for use with certain activators (e.g., ionic borates).
The heterocyclobutenyl anion is preferably generated at low temperature (0xc2x0 C. to 100xc2x0 C.), preferably in an inert solvent (e.g., a hydrocarbon or ether). The anion is then usually added to a solution of the transition metal source at low to room temperature. After the reaction is complete, by-products and solvents are removed to give the desired transition metal complex.
In another approach to making the complex, a synthetic equivalent of a heterocyclobutenyl anion reacts with the Group 3-10 transition metal source. By xe2x80x9csynthetic equivalent,xe2x80x9d we mean a neutral compound capable of generating an anionic heterocyclobutenyl ligand under the reaction conditions. Suitable synthetic equivalents include heterocyclobutenes that are C-substituted with xe2x80x94QRxe2x80x33 groups, where Q is Si, Sn, or Ge, and Rxe2x80x3 is a C1-C30 hydrocarbyl group. When combined with suitable transition metal sources, particularly ones that have a labile anionic group such as halide or dialkylamino, a complex incorporating a heterocyclobutenyl anion is produced with elimination of a neutral Sn, Ge, or Si-containing by-product. Usually, it suffices to combine the synthetic equivalent and the transition metal source in a suitable solvent and heat if needed to complete the reaction. For example: 
The catalysts are particularly valuable for polymerizing olefins. Preferred olefins are ethylene and C3-C20 xcex1-olefins such as propylene, 1-butene, 1-hexene, 1-octene, and the like. Mixtures of olefins can be used. Ethylene and mixtures of ethylene with C3-C10 xcex1-olefins are especially preferred.
Many types of olefin polymerization processes can be used. Preferably, the process is practiced in the liquid phase, which can include slurry, solution, suspension, or bulk processes, or a combination of these. High-pressure fluid phase or gas phase techniques can also be used. The process of the invention is particularly valuable for solution and slurry processes. Suitable methods for polymerizing olefins using the catalysts of the invention are described, for example, in U.S. Pat. Nos. 5,902,866, 5,637,659, and 5,539,124, the teachings of which are incorporated herein by reference.
The olefin polymerizations can be performed over a wide temperature range, such as about xe2x88x9230xc2x0 C. to about 280xc2x0 C. A more preferred range is from about 30xc2x0 C. to about 180xc2x0 C.; most preferred is the range from about 60xc2x0 C. to about 100xc2x0 C. Olefin partial pressures normally range from about 15 psia to about 50,000 psia. More preferred is the range from about 15 psia to about 1000 psia.
Catalyst concentrations used for the olefin polymerization depend on many factors. Preferably, however, the concentration ranges from about 0.01 micromoles per liter to about 100 micromoles per liter. Polymerization times depend on the type of process, the catalyst concentration, and other factors. Generally, polymerizations are complete within several seconds to several hours.